The other primary contenders pledged to unite their forces behind Capriles in an attempt to unseat Chávez, who has held the Venezuelan presidency for the last 13 years.

Capriles, the 39-year-old governor of Miranda state, faces a tough task in ousting Chávez, a charismatic campaigner with a loyal following and the full powers of the state to back his candidacy in Oct. 7 elections.

Opposition election chief Teresa Albanes announced the preliminary results, saying that Capriles won about 62 percent of the vote, beating Zulia state Gov. Pablo Pérez by a margin of more than 30 percentage points.

Thousands of supporters celebrated the win outside Capriles' campaign headquarters, some holding small flags bearing the slogan "There is a way." Fireworks exploded in the sky overhead.

"I aim to be the president of all Venezuelans," Capriles told the crowd, wearing a baseball cap emblazoned with the yellow, blue and red of Venezuela's flag. "It isn't the time of lefts or rights. It's the time of all Venezuelans."

His four defeated rivals promptly united behind Capriles and joined him on the outdoor stage. Standing side-by-side, they grasped hands and raised them.

"In union there's strength!," Capriles shouted.

Some of Capriles' supporters say they think he has a good chance of winning over Venezuelans who otherwise might lean pro-Chávez because he has taken a largely non-confrontational approach toward the president while promising solutions to problems including 26-percent inflation and one of the highest murder rates in Latin America.

Diego Prada, a 23-year-old marketing manager, said he thinks Capriles' inclusive approach offers a much better shot against Chávez than other competitors who have taken a hard line against the president.

"People are tired of so much confrontation," Prada said. And Capriles, he said, offers "a message of unity."

The once-divided opposition has gained popularity in recent years, and the race could end up being the toughest re-election bid of Chávez's career.

The leftist president easily won re-election with 63 percent of the vote in 2006, but since then his popularity has declined, in part due to ills including crime and economic troubles.

Chávez's approval ratings have topped 50 percent in recent polls, and his struggle with cancer doesn't appear to have hurt his popularity. The president says he's cancer-free after undergoing surgery and chemotherapy last year, and has been energetic in his hours-long television appearances, apparently trying to show he can still keep up with a younger challenger.

Chávez remains a hero to many of his supporters and maintains a visceral connection to a significant segment of the poor in Venezuela. He also will use the full powers of his government and a bonanza of public spending to try to ensure a victory in the Oct. 7 election.

Chávez has already kicked his campaign machinery into gear. He has increased government spending by launching new social programs that offer cash benefits for the poor and invested heavily in new railways, public housing and cable car systems in Venezuela's sprawling hillside slums.

As the election nears, he will inaugurate other big-ticket projects that grab attention, including the planned launch of Venezuela's second Chinese-made satellite shortly before the October vote.

But Chávez has warned voters that if they don't re-elect him, his social programs called "missions" would vanish. That threat, though disputed by Capriles, could have an influence on some in the run-up to the vote.

For the opposition, the primary showed its ability to mobilize voters, a key asset in its efforts to compete with Chávez. Lines formed on Sunday at polling stations in some poor neighborhoods that have traditionally been pro-Chávez strongholds.

"I decided to come to vote to express my complete unhappiness. In these 12 years, the country has gone downhill," said Rubén Rodríguez, a 59-year-old construction worker who was waiting in line to vote.

Capriles is a moderate who says he is an admirer of the approach of former Brazilian President Luiz Inácio "Lula" da Silva.

In order to compete, Capriles probably will need to win over voters who leaned pro-Chávez in the past, who have grown disillusioned with the government and who don't strongly identify with either side.

"Confrontation and fear are going to be part of that past," Capriles said after voting. "Hope can deal with any obstacle they put in our way. Today is a day of hope."

Candidacies for other posts including state governors were also being decided in the primaries. Venezuelans living abroad were able to cast ballots in cities from Miami to Madrid.

Venezuela has grown heavily polarized, with most either admiring or despising Chávez. About one-fourth of voters are in neither political camp, and in that group about 10 to 15 percent are likely to cast ballots, said pollster Luis Vicente León. Many of the swing voters are young people who have grown up during Chávez's presidency, León said.

Recent polls before the primary vote showed Capriles with about 40 percent support among opposition voters.

Mercedes Aponte, a 60-year-old high school teacher, said she's convinced Capriles would bring improvements in education, health care and anti-crime efforts.

"Through him, there's hope. It's a new day dawning for Venezuela," Aponte said, waiting to vote in a line that snaked around the block in downtown Caracas.

Capriles might not be able to compete with Chavez's government money nor the president's ability to take over the airwaves of all TV and radio stations when he deems appropriate. But Capriles can count on ample campaign funding from anti-Chávez donors, as well as high visibility in opposition-aligned media including the television channel Globovision, private radio stations and newspapers.

The country's opposition coalition, which united to hold a presidential primary vote for the first time, has become better organized and will be an important ally in mobilizing voters for Capriles against Chávez's campaign machine.

The results were announced with 95 percent of ballots counted, and Capriles dominated the field with more than 1.8 million votes. Several of the opposition contenders called the higher-than-expected turnout a victory.

Opposition presidential contender Maria Corina Machado, a congresswoman who takes a hard line against Chávez, conceded defeat before the results were announced, saying she also will actively back Capriles' campaign.

"We will stay united," she said. "Tomorrow, the struggle intensifies."